Acoustic logging systems



Dec. 13, 1966 P, MAJANI ETAL 3,2%,248

ACOUSTIC LOGGING SYSTEMS Original Filed Jan. 26, 1960 2 Sheetsheet l 6/#a4/r @ma 99 V M T f 47 Y fafa. Hf@

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ACOUSTIC LOGGING SYSTEMS Original Filed Jan. 26. 1960 2 Sheets-Sheet 2/D/'e rf e Maja/7 /af? l/. HvJ/7 fof? 1N VEN TORJ` ATTORNEY tates Thisapplication is a continuation of co-pending plication Serial No. 4,809,filed January 26, 1960, Acoustic Logging Systems, now abandoned.

rThe present invention relates to acoustic logging apparatus and, moreparticularly, to acoustic apparatus for investigating acoustic couplingof materials to a casing in a borehole.

In a typical well completion operation, a casing is positioned in theborehole and cement pumped into the annulus defined between the casingand borehole. For various reasons, the cement may fail to completelytill the annulus along the length ofthe casing, which often results in acondition generally known as channeling Moreover, part or portions ofthe column of cement may fail to bond to the casing or formations. Insuch instances, uid from formations at other depths may migrate alongthe imperfections in the column of cement. Thus, if a production sand iscompleted as by perforation in a zone which includes such imperfections,migrating fluids and/or gases from other earth formations may impair theproduction of hydrocarbons from the sand.

In another typical instance, recovery operations for a stuck drill pipe,casing or the like wedged tight in a borehole by the earth formationsoften require a knowledge of the depth of the stuck point.

By means of the present invention, in the above instances, the acousticcoupling of material to the pipe string in the borehole, either cementbonding or the earth formations packing about the casing, isinvestigated to provide the desired information.

Accordingly, it is an object of the present invention to provide new andimproved methods of determining locations behind a casing in a boreholeat which materials are acoustically coupled to a casing.

An additional object of the present invention is to provide new andimproved apparatus for determining by acoustic velocity techniques, thequality of a cement bond behind a casing.

in the systems embodying the present invention, to investigate thecement bonding of a casing in a borehole, acoustic energy is generatedat a first location centrally of the casing and passed along the casingto a second location centrally of the casing where it is intercepted andtranslated into an electrical signal representative of the acousticenergy. Signal discrimination is employed with respect to the relativeampiitude of a signal representing the acoustic energy passing along thecasing wherein the casing or formation is not bonded to cement or stuckby other materials and the amplitude of a signal representing acousticenergy passing along the casing wherein the casing is bonded to cementor stuck by other materials. in this manner, a record of indications indeveloved with respect to depth to provide indications where the cementdoes not bond to the casing. This discrimination is possible becausesignals representative of acoustic energy may be developed whichtypically consist of a series of alterapfor dili Patented Dec. 13, ISGSnations having a characteristic rst cycle with a first peak of smallamplitude and a given polarity relative to a reference value, followedby a second peak of considerably larger amplitude peak with an oppositepolarity. The immediately following cycles of the electrical signalgenerally have peak amplitudes yof opposed polarity which are relativelymuch larger than either the rst or second peaks. Where the casing isfree, that is, the cement fails to bond or the spacing between thecasing and borehole is open, the casing freely transmits the acousticsignal so that the amplitude of the second peak, for example, ispredictable. On the other hand, if cement is bonded to the casing or, ininstances when the formation is tightly packed about the casing, theacoustic energy passing through the casing is very greatly attenuated,so that the amplitude of the second peak is decidedly decreased. Hence,by developing indications of occasions where the second peak achieves apredicted value for free casing, it can be determined that cement hasfailed to bond to the casing and that it is free.

FIG. 1 illustrates a suitable apparatus for carrying out the method ofthe present invention; and

FIGS. 2a, 2b are representations of certain signals useful in explainingthe present invention.

In FIG. l of the drawings, an apparatus for performing the method ofinvestigating the quality of cement bonding between a casing and aborehole as well as determining free casing is shown to comprise aborehole instrument including an upper, electronic section 8 and a lowertransducer section 9 enclosed by appropriate pressure-tight housingscapable of withstanding hydrostatic pressures encountered in a casing 10where theunit is suspended by means of an armored, electric cable 11.Casing 10 is shown set in a borehole 12 and intermediate of the borehole12 and casing 10 is an annulus 13 which may be cement. Should the cementfail to fill all of the annulus, earth formation spalling may pack aboutthe casing or mud may be in the annulus. The casing l0 and boreholetraverse the earth formations. By means of the cable 11 and aconventional winch (not shown), the borehole instrument may be passedthrough the casing l() in order to obtain useful information concerningcharacteristics of free casing and the cement bonding to the casing 1).Casing 1t) is filled with a fluid medium 15 which completes a soundtransmission path between the transducer section 9 and the casing 10.The borehole instrument is centered in the casing 10 by means ofconventional centralizers (not shown) to locate the transducer sectionalong the central axis of the casing.

Eletrical energy from an alternating current generator 16 at the surfaceof the earth is supplied via conductors 17 and 1S of cable Il andappropriate ground connections to a conventional power supply il? withinelectronic section S. The power supply 19 convelts the appliedalternating current to unidirectional potentials of appropriatemagnitudes for operating the various circuit elements within electronicsection 8 and in section 9.

A master keyer 2t) which may be a conventional freerunning multivibratoror a multivibrator synchronized with the frequency of source 16 suppliesrepetitive master keyer pulses at millisecond intervals, for example,over a conductor 2l. to a conventional pulser 23. In

synchro-mism with each applied pulse, pulser 23 supplies a pulse of highcurrent having a duration of, for example, one microsecond over leads 24to a transmitting transducer 25 which may be of the magnetostrictiontype.

Preferably, both the pulser 23 and the transducer 25 are located in theupper end of transducer section 9.

To derive electrical signals in response to acoustic energy, a firstreceiving transducer 26 and a second receiving transducer 27 arepositioned below the transmitter 25 in the recited order. The receivers26 and 27 may be of the magnetostriction type and may be spaced from oneanother approximately three feet. The upper receiver 26 also may bespaced from the transmitter 25 .a distance of three feet. Thesedistances, however, may be set at any desired values. Alternatively, byusing one or more additional receivers, multiple spacings may beemployed on the same trip into a borehole.

The receivers 26 and 27 are coupled by leads 28 and 29 to respectivepulse .amplifiers 30 and 31 whose output circuits are coupled toindividual .-gated pulse generators 32 and 33. These pulse generatorsare of conventional construction arranged so that no output signal isproduced unless a control pulse is supplied to a control circuit.

To derive control pulses for generator 32, an extension of lead 21 frommaster keyer 20 is connected to the input circuit of a delay circuit 34which provides a delay of approximately 140 microseconds. Delay circuit34 is connected by a lead 35 to a conventional gate multivibrator 36which generates a pulse of approximately 700 microseconds duration thatis supplied via a lead 37 to the control circuit of pulse generator 32.A gate multivibrator 38 that is similar to gate multivibrator 36 has itsinput circuit connected by a lead 39 to the output circuit of pulsegenerator 32, and its output circuit is connected by a lead 40 to thecontrol circuit of gated pulse generator 33. The purpose of this type ofconnection will be more apparent from the discussion to follow.

Output lead 39 of generator 32 and -output lead 41 of generator 33 areconnected to respective input circuits of a` conventional multivibrator42 which provides a pulse whose duration is dependent upon the timeinterval between the pulses applied to its input circuit. To derive anindication of travel time, the output circuit of multivibrator 42 isconnected by a lead 43 to a constant current devi-ce 44 arranged tocharge a condenser 45 in its output circuit in relation to the durationof each applied lpulse. Master `keyer is connected by leads 20a to theoperating coil 20b of a relay having normally open contact 20cconnectedyacross condenser 45. The signal at leads Ztla is arranged tobe in the form of a pulse which begins approximately 70 millisecondsafter each master keyer pulse and of 30 milliseconds in duration.Accordingly, the condenser 45 is short circuited and is thus dischargedand so remains during the 30 millisecond interval preceding each emittedpulse. At the condenser 45 there thus Aappears -a pulse signal whoseamplitude is dependent upon the duration of the pulse supplied bymultivibrator 42. The condenser is connected to an output circircuitcomprised of a conventional amplifier and cathode follower (not shown),in turn, connected Iby lan insulated conductor 47 of cable 13 andappropriate ground connections to a conventional peak-reading voltmeter48 at the surface of the earth. The voltmeter output is connected byleads 49 to a recorder 50 in which, through a conventional linkage 51and a measuring wheel 52, the recording medium is transported inproportion to movement of cable 11.

In the performance of the method, in accordance with the presentinvention, gated pulse generators `32 and 33 are employed of the typethat produce output pulses only in response to the application of inputpulses or signals exceeding a given bias or voltage level. Adjustment ofthe bias level on these generators may be accomplished in a conventionalmanner; for example, a lead 60 extends from the surface control throughcable 11 to the bias-control circuit (not shown) of generator 32. Thebias voltage to generator 32 thus may be adjusted by means of apotentiometer 61 connected in parallel vtdth a battery 62. In order toobtain a record of the voltage supplied rto the Cil control circuits ofgenerator 32, the potentiometer 61 is connected by leads 63 to recorder50. Similarly, a lead 60 extends from the surface through cable 11, tothe bias-control circuit (not shown) of generator 33. The bias voltageto generator 33 thus may be adjusted by means of a potentiometer 61connected in parallel with a battery 62. In order to obtain a record ofvoltage supplied to the control circuits of generator 33, thepotentiometer 61 is connected by leads 63 to recorder 50.

In operation, the units 8, 9 :are lowered in the casing by means ofcable 11 and, as it is subsequently raised, repetitive pulses areemitted from transmitting transducer 25 `and propagated through thefluid medium 15 to the casing 1t).

The pulse on lead 21 which operates pulser 23 thereby to generate atransmitted pulse is also supplied to delay circuit 34 and 14()microseconds later a pulse triggers multivibrator 36. The multivibrator36 provides a negative-going pulse whose leading edge is synchronizedwith the pulse from delay circuit 34 and whose trailing edge occurs 700microseconds later. This control pulse is supplied over lead 37 tocondition pulse generator 32 for operation. Accordingly, when the signalrepresenting a first arrival pulse of acoustic energy passing throughthe cassing is supplied by receiving transducer 26 to the amplifier 30,the amplified pulse causes generator 32 to generate 'an output pulse atlead `39. This plulse triggers multivibrator 38 and the resultingcontrol pulse having its leading edge synchonized with the pulse fromgenerator 32 and its trailing edge occurring 700 microseconds later, isapplied over lead 40 to the control circuit of pulse generator 33.Generator 33 is thus operatively conditioned and when the signalrepresenting Aacoustic energy incident upon receiving transducer 27 istranslated by amplifier 31, generator 313 is triggered. The pulses fromgenerators 32 and 33 are supplied to multivibrator 42 which generates apulse having a duration representing the time spacing or interval Atbetween the applied pulses. The latter pulse is supplied to constantcurrent device 44.

Constant current device 44 causes the condenser 45 to charge linearlyand this occurs for the duration of each pulse from the multivibrator42. Accordingly, the condenser attains a charge voltage when isproportional to the duration of applied pulse. Thirty millisecondsbefore the next cycle of operation begins, the pulse at leads 20aenergizes coil 2Gb thereby closing contact 20c and the condenser isdischarged. These contacts open just prior to the next cycle ofoperation.

It is evident that the charge voltage on condenser 45 represents thetime interval (At) between pulses received at transducers 26 and 27 and,of course, the reciprocal of this quantity represents acoustic velocity.Circuit resistance across the condenser is lcept to a maximum so thatthe condenser remains at the particular charge Voltage un-til it isshort circuited by contacts 20c. The foregoing cycle is repeated witheach master keyer pulse and voltage pulses are developed at condenser 45of amplitudes representing travel time. These pulses are supplied Viathe output circuit 46 and cable conductor 47 to the peak-readingvoltmeter 48 and the resulting voltage is supplied to the recorder 50.Thus, a log is derived representing the 4transit time (or acousticvelocity).

To facilitate an understanding of the present invention, it should beunderstood that when the cement 13 bonds to the casing, rthe pulse ofenergy from the pulser 23 transmitted through the casing is very greatlyattenuated in amplitude. This results in an acoustic signal arriving atreceiver 26 with a relatively low energy level or amplitude ascontrasted to the energy level of the acoustic signal transmittedthrough a casing which does not have a cement bond. In either case, theacoustic energy which arrives at the second receiver 27 is even furtherattenuated relative to the energy which arrives at the irst receiver 26.

Referring now to FIG. 2, the signal conditions are illust'rated bywaveforms (not to scale) which further illustrate concepts of thepresent invention. As shown in FIG. 2a, a relatively large a-cousticalsignal arriving at receiver 26 produces a large electrical signal whenthe casing is free or imperfectly cement bonded, the electrical signalbeing represented by a solid line curve 65 originating at a time t0. Theenergy represented by amplitude of the first negative swing of thesignal 65 is greater than or exceeds a given bias level 66 set at avoltage value of El. The bias level 66 is represented by a magnitude Elon the lower or negative portion of the amplitude scale for convenienceof explanation although the bias actually may have a positive polarity.Thus, the amplitude of the casing signal, when the casing is imperfectlybonded to the earth formations, exceeds the bias level 66 and triggersthe pulse generator 32 at a time t1.

As shown in FIG. 2b, the same casing signal 65 arrives at receiver 27 ata subsequent time r3 as shown by waveform 65 and is somewhat attenuated;however, the amplitude of the first negative swing of the signal exceedsthe bias level 66 so that generator 3-3 produces a pulse to actuatemultivibrator 42 at a time t4. The multivibrator 42 and co-nstantcurrent device 44 being turned on at t1 and off at 14 thereby producesan indication of the time interval Ar between the signals 65, 65respectively developed by the receivers 26 and 27 by developing avoltage pulse on condenser 45 in the manner described heretofore. The Atmeasurement for a steel casing with a span of three feet betweenreceivers would, for example, be 171 microseconds or, on a per footbasis, 57 microseconds.

The At `measurement is then applied to a record in the recorder Si),which is correlated with the depth at which the measurement was taken.For an interval of casing passed by the apparatus, the various Az*measurements would be constant so long as the casing remained free ofcement or formations. However, the casing collars coupling the sectionsof casing to one another provide an anomaly in the string of casingwhich is readily detectable due to changes in attenuation of theacoustic signal so the casing collars are indicated at regular intervalsby slight At measurement changes.

In an interval of the casing along which cement is bonded to the casing,the acoustic signal would be attenuated to a greater extent. Thus, asignal 67 (FIG. 2a) would be developed by receiver 26. It will be notedthat the bias level 66 is adjusted so that the rst negative swing ofeither signal 65 4or 67 will trigger the generator 32. Hence, signal 67will trigger the generator at a time t2 which `is substantially the sameas time t1. Iowever, when the acoustic energy arrives at the secondreceiver, the signa-l 67 is attenuated so that the iirst negative peakfails to achieve the bias vol-tage level 66 `so that the generator 33 isnot triggered until the second negative peak exceeds level 66 `at a timet5. Thus, the time interval between the detecting times at t2 and f5will be greater than the time interval between the detecting times r1and t4 by one cycle of the signal. With a transmitter frequency of 30kc., for example, the period would be 33 microseconds per foot. Hence,on the recording for a three foot spacing between receivers, anindication of 270 microseconds (1714-99) indicates the presence andbonding of cement to the casing.

From the foregoing, it will be appreciated that as the apparatus ispassed along the casing, a record is developed which includes a baseline indicating the velocity of acoustic energy where the casing is freeof bonding together with excursions to one side of the base line basedon a definite time period at intervals along the casing where bonding ofcement or packing of formations to the casing occurs.

In performance of the method, the bias voltage is set to a level wherethe At measurement obtained is representative of the time intervalnormally obtained for the passage of acoustic energy through casing. Toset the bias level, the apparatus is lowered to a location where thecasing is known to be free. Thereafter, the apparatus is relocated at alocation where the casing is known to have cement bonded thereto. Then,the bias level is readjusted to obtain a At measurement which is afunction of the above-'mentioned time interval for passage of acousticenergy through the casing plus a time interval representative of one ormore additional alternations of the acoustic energy. The setting of thebias level may be done with the apparatus in the various positions alongthe casing, as above described, or may be a predetermined valuedetermined from experience. In any event, the level is a value which isslightly less than the expected amplitude of an attenuated signal at thereceiver nearest the transmitter and greater than the expected amplitudeof the attenuated signal at the receiver farthest from the transmitterwhen the receivers are opposite cement bonded casing. Also, it should benoted that centralizing of the apparatus in the casing is necessary toinsure the uniformity of casing signais as well as their maximumintensity.

The magnitude E2 of the bias level 66' may be adjusted to be equal tothe magnitude El by appropriate adjustments of potentiometers 61, 61'.However, by the independent adjustment of bias level 66', the magnitudeE2 may be increased, for example, to the level 66 which still allowsdetection of free casing but increases the reliability for detection ofstuck or bonded casing in that a larger amplitude of signal is requiredto actuate the pulse generator.

A cement top may be easily located by the use of the present inventionby observing the transition of the At from a greatly attenuated acousticsignal obtained in stuck or bonded casing to a normal At indication fora free casing. The transition occurs when the cement top is reached andthe acoustic energy is no longer attenuated. Thus, the energy isconfined to the casing since no cement bonding is present to attenuatethe casing signal and the At indication is constant.

Vhile only one apparatus is herein described, another apparatus forperforming the method is disclosed in the copending .application of LeeH. Gollwitzer, Serial No. 831,- 328, tiled August 3, 1959.

While particular embodiments of the present invention have beendescribed, it is apparent that changes and modifications thereof may bemade without departing from the invention in its broader aspects, andtherefore, the aim in the appended claims is t-o cover all such changesand modications as fall within the true spirit and scope of theinvention.

What is claimed is:

1. The method of investigating the quality ofthe bonding of an annulusof cement to a metallic casing which has been cemented in `a boreholecomprising the steps of: probing intervals of the casing with acousticenergy traveling between spaced locations; setting the energy level atwhich acoustic energy is detected to obtain sensible indications of thetravel time of acoustic energy through the casing between the spacedlocations at an interval Ialong the bore where the casing is not bondedto the cement; re-setting the energy level at which acoustic energy isdetected to obtain sensible indications of the travel time of acousticenergy through the casing between spaced locations at an interval `alongthe bore where the casing is bonded to the cement; and thereafterfinally setting the energy level at which acoustic energy is detected toobtain sensible indications differentiating between the travel time ofacoustic energy between said spaced locations through a cement bondedcasing and a casing which is not bonded to the cement.

2. The method of locating zones of bonding between wel] pipe andsurrounding material with the aid of acoustic energy transmitted alongthe well pipe, the acoustic energy attenuation characteristic of thewell pipe being dependent upon the quality of the bond to thesurrounding material, the better the bond the greater the attenuation,comprising the steps of generating pulses of acoustic energy atintervals along a Well pipe substantially along the axis thereof andtransmitting at least a part of said energy along said Well pipe,detecting the arrival of such acoustic pulses at a pair of spaced pointsspaced substantially a xed dist-ance along said aXis from the points ofgeneration yto develop a corresponding pair of electrical signals,detecting only excursions of said electrical signals which occur attimes following generation `of a pulse representative of the traveltimes of acoustic energy along the pipe, deriving from said excursionsfurther electrical signals indicative of the attenuation of saidacoustic energy provided by said well pipe, and `recording said furtherelectrical signals as a function of position along the pipe to providean indication of t'he quality of the bond between the pipe and thesurrounding mate-rial.

References Cited by the Examiner UNITED STATES PATENTS BENJAMIN A.BORCHELT, Primary Examiner.

CHARLES W. ROBINSON, CHESTER L. IUSTUS,

SAMUEL FEINBERG, Examiners.

R. I. BAYNHAM, A. E. HALL, I. W. MILLS, R. M.

SKOLNIK, Assistant Examiners.

1. THE METHOD OF INVESTIGATING THE QUALITY OF THE BONDING OF AN ANNULUSOF CEMENT TO A METALLIC CASING WHICH HAS BEEN CEMENTED IN A BOREHOLECOMPRISING THE STEPS OF: PROBING INTERVALS OF THE CASING WITH ACOUSTICENERGY TRAVELING BETWEEN SPACED LOCATIONS; SETTING THE ENERGY LEVEL ATWHICH ACOUSTIC ENERGY IS DETECTED TO OBTAIN SENSIBLE INDICATIONS OF THETRAVEL TIME OF ACOUSTIC ENERGY THROUGH THE CASING BETWEEN THE SPECEDLOCATIONS AT AN INTERVAL ALONG THE BORE WHERE THE CASING IS NOT BONDEDTO THE CEMENT; RE-SETTING THE ENERGY LEVEL AT WHICH ACOUSTIC ENERGY ISDETECTED TO OBTAIN SENSIBLE INDICATIONS OF THE TRAVEL TIME OF ACOUSTICENERGY THROUGH THE CASING BETWEEN SPACED LOCATIONS AT AN INTERVAL ALONGTHE BORE WHERE THE CASING IS BONDED TO THE CEMENT; AND THEREAFTERFINALLY SETTING THE ENERGY LEVEL AT WHICH ACOUSTIC ENERGY IS DETECTED TOOBTAIN SENSIBLE INDICATIONS DIFFERENTIATING BETWEEN THE TRAVEL TIME OFACOUSTIC ENERGY BETWEEN SAID SPACED LOCATIONS THROUGH A CEMENT BONDEDCASING AND A CASING WHICH IS NOT BONDED TO THE CEMENT.